Relativistic Laser-Plasma Physics
When a UHI laser hits a solid foil it quickly ionizes the front surface. The electrons move in the combined laser and plasma electromagnetic fields. Due to the strong fields the electrons rapidly become highly relativistic. This can give rise to rich physics, such as instabilities (e.g. Rayleigh-Taylor (RT) like, Parametric, Weibel, Buneman, Resistive and Ionization instabilities), shock formation, buried layer heating by internal ambipolar expansion. These effects are important for example for an understanding of almost all fundamental questions in laser-solid plasma physics, such as laser absorption, electron and ion acceleration (RPA, TNSA, BOA), filamentation of electron and ion beams at the foil surface, inside the foil or behind the foil, and the generation of high harmonics. However, there does not yet exist any direct experimental observation of any of the mentioned processes (though e.g. indirect observations of hole boring via Doppler shift of reflected light exist). The reason is that the timescale of the physics is ultra-short, on the order of the pulse duration (few 10 femtoseconds), UHI laser period (typically few femtoseconds) or the plasma period (below 0.1 femtoseconds) and the relevant spatial scales are generally in the range of a few microns and below. Moreover, solid foils are not penetrable by IR, visible, or UV light.
Developing a predictive understanding of these, together with their interactions, and also far from equilibrium is a grand challenge of modern-day plasma physics, which will require significant advances in theory and numerical simulation of non-equilibrium and non-linear processes. At the same time it is of the utmost importance to obtain much better experimental data with high spatial and temporal resolutions. Probing the solid-density plasma with XFELs will open entirely new ways to directly observe these extreme conditions, and will provide fundamentally new data for developing improved models, as well as validate or falsify present theoretical treatments. This is the prerequisite to drive advanced applications such as fast ignition or ion acceleration to > 100 AMeV energies. Key observables include the local electron density, current density, quasi-static magnetic fields, and ionization state, as well as the growth rate of the various perturbations and instabilities.